Flexible vertebral linking device

ABSTRACT

A flexible intervertebral linking device ( 1 ) is provided. The device ( 1 ) utilizes two sets of structures. A first structure ( 11 ) is a rigid structure ( 110, 112, 114, 116 ) preferably made of biocompatible metallic materials providing the device with good mechanical resistance by integral load transmission without deformation. A second structure ( 12 ) is a flexible or damping structure ( 121  and  122 ) made of biocompatible viscoelastic materials, permitting repeated elastic deformations. The combination of the two structures providing the device with both resistance and mechanical dampening of forces to which it is subjected, with the purpose of compensating for any deficiency in the flexibility of certain anatomical links of the human body.

FIELD

The invention relates to a posterior flexible vertebral linking devicewhich works in tension, compression and flexion, and which dampens allmechanical stresses. This device has operational advantages that will bedescribed.

BACKGROUND

Many posterior vertebral attachment units exist which rigidize a certainnumber of vertebrae by depriving them of any mobility, thus containingall mechanical stresses. However, the first vertebra adjacent to thisrigidized block of vertebrae remains mobile and, consequently, there isan abrupt discontinuity in movement between the rigid block and thisfree vertebra which very often generates a very high stress in thelinking elements. The result is an acceleration of the degeneration ofthis level (the interface between the adjacent vertebrae and thevertebrae comprising part of the rigidized block).

This problem was only partially solved by semi-rigid systems conceivedto create an intermediate rigidity between the mobile vertebrae and thefixed vertebrae. These prior art systems present primarily one of twodisadvantages, either they work only in tension or they work incompression with a thrust in tension.

As for those that work only in tension: this is the case of all thedevices based on artificial ligaments. These systems have littleelasticity and leave the care to regulate, in particular, the tension,to the skill of the operator, thus making the mechanical characteristicsin the operating mode of interest (tension/compression) haphazard.

Further, those devices that work in compression with a thrust in tensionare ineffective when dealing with displacements in tension.

In either case, none of the known devices entirely solves the problemwhich is posed, namely, damping the mechanical stresses existing intension/compression and flexion to which a moving vertebra is subjected.

Patent Application EP 0576 379 A1 proposes a shock absorber which seemsto approach most closely at least from the point of view of the generalconcept of this invention. This patent describes a uni-axial shockabsorber working only in compression while playing the part of anabutment which opposes any displacement of the piston beyond a givenvalue.

The invention of EP 0576379 A1 deals with the exponential limitation ofthe displacement, which is a completely different problem as that of thepresent invention.

French Patent application No. 0,012,998, describes and claims a flexibleand cast solid vertebral linking device functioning in amultidirectional way. This prior art reference does not solve exactlythe same problem as that of the present invention due to its differentmeans and functions.

SUMMARY

The invention concerns a flexible intervertebral linking device thatmeets the needs identified above. The device (1) utilizes two sets ofstructures. A first structure (11) is a rigid structure (110, 112, 114,116) preferably made of biocompatible metallic materials providing thedevice with good mechanical resistance by integral load transmissionwithout deformation. A second structure (12) is a flexible or dampingstructure (121 and 122) made of biocompatible viscoelastic materials,permitting repeated elastic deformations, the combination of the twostructures providing the device with both resistance and mechanicalstress damping of forces to which it is subjected, with the purpose ofcompensating for any deficiency in the flexibility of certain anatomicallinks of the human body.

In an advantage, the surgeon can choose in a precise way the desiredworking method: tension/compression or flexion, or the combination ofthe two working methods, so as to avoid any contact between thearticular facets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are perspective views (two alternative embodiments) ofthe device operating in tension, compression and flexion.

FIGS. 2A and 2B are longitudinal cross-sectional views of twoalternative embodiments of the invention.

FIG. 3 is an exploded view of the invention.

FIG. 4 is a perspective view of the invention for operation only intension/compression.

FIG. 5 is a cross-sectional view of the invention operating only intension/compression.

FIGS. 6 to 11 are perspective views depicting the individual parts ofthe invention.

FIG. 12 is a perspective view of another component for operation in thetension/compression mode.

FIG. 13 is a side, cross-sectional view of an alternative of the deviceworking along two axes.

FIGS. 14 to 17 are perspective views of four forms of the mobile end ofother embodiments of the invention.

FIG. 18 is a side, cutaway view of the invention in place in a patient.

DETAILED DESCRIPTION

The device 1 utilizes two sets of structures. A first structure 11 arigid structure manufactured out of preferably metal, biocompatiblematerial ensuring a good mechanical resistance of the device bycompletely transmitting the forces.

A second structure 12 is a flexible or damping structure manufacturedout of viscoelastic biocompatible materials, supporting the repeatedelastic strain. It is the combination of these two structures whichmakes the operation of the invention possible.

The first structure 11 includes four mechanical structures 110, 112,114, 116 which have the function of transmitting stresses to which thedevice 1 is subjected without deformation.

The mechanical structure 110 is made up of a mechanical rod 111, one ofits ends being surmounted by a circular plate 113 b connected to the rod111 with a broad joining radius 113 a, and the assembly being able toslide in the cavity of the structure 114 which encloses a visco-elasticelement 121.

The mechanical structure 112 is a cap provided with a thread 117allowing for the fixing of the structure 112 on structure 114; thestructure 112 has a shoulder area 118 which encloses aviscoelastic-centering ring 121 between the plate 113 b.

The mechanical structure 114 is made up of two hollow cylinders, one ofwhich is tapped to allow the fixing of a rod 116 with a threaded end.The mechanical structure 110 and 116 will be fixed on the vertebrae topermit the functioning of the device 1.

The second structure 12 is made up of two viscoelastic components 121and 122.

The centering ring 121 preferably allows the rod 111 to slide in itscenter.

The second component 122 is a disc of viscoelastic material. These twocentering rings 121 and 122 can undergo compressive stresses which maynot be uniformly distributed. They are formed to resist many cyclicfatigue stresses without breaking. Compressive tests have been performedwhich verify that the components 121 and 122 are able to undergone thesetests of elastic deformation as many times as necessary.

The selected material is preferably a biocompatible polyurethane; thanksto their integration inside mechanic components 110, 112, 114, 116, theviscoelastic elements 121 and 122 are protected by the precedingmechanical structures from the aggressive environment of the human body,which avoids in particular the formation of fibers around thesecomponents which could deteriorate the viscoelastic properties of thematerial and consequently disturb the correct operation of device 1.

This device 1 makes possible the damping of the stresses intension/compression and flexion which it undergoes by the intermediaryof rods 110 and 116. This function is assured owing to the fact thatcomponent 112 has a sufficiently broad opening 119 to allow a clearanceof rod 111 and that there is a functional allowance between plate 113and the hollow body of component 114; the shoulder area 118 serves as astop and maintains in its housing the viscoelastic element 121 thusconstrained.

If one wishes to work in a uni-axial mode of tension/compression,component 112 is replaced by another component 115 equipped with threads117, which includes a cap 115 c, whose opening 119 is of a diameterwhich corresponds to the diameter of the rod 110 and which is elongatedwith a rod guide 115 a.

This device 1 is thus able to react dynamically to the applied stresses.Note that it is essential that structure 114 comprises a bore 114 a toallow for low friction guidance of rod 110 in the aforementionedcomponent 114.

The adjustment of the diameter of the viscoelastic centering rings 121and 122 must be selected with precision to enable them to be crushedfreely until a stress threshold is reached, this threshold correspondingto a point of contact of the bore 114 a of component 114.

An alternative to the structure 11 includes metal structures having thesame functions as the structures 110, 112, 114, 116, but the assembly ofthese three parts (110,130, 131) having a lower threshold than that ofthe structures previously described (see FIG. 2).

The rod 131 is fixed at its cap 130 by the intermediary of a threadlocated on shoulder 132 of the rod.

In this alternative embodiment, the possibilities of displacement of rod110 subjected to the stresses in flexion are enabled by play 119 locatedbetween cap 130 and rod 110.

For a uni-axial operation of device 1, it is preferable to usecomponents 110,112, 114, 116 which provide a better guidance of rod 110.If small overall dimensions are needed, components 110, 130, 131 may bepreferably used.

Device 1 is able to function with rods 110 and 131 moving on convergentaxes (FIG. 13) with a small angle of displacement and according to givenclearances.

The structure 12 is therefore comprised of two visco-elastic components141 and 142. Note that the rod 110 has a flange 110′ on its end. Thecomponent 141 is a cylinder of visco-elastic biocompatible materialwhose face in contact with the flange 110′ is inclined. The component142 is a centering ring whose face in contact with the back of theflange 110′ is inclined.

The structure 11 (rigid means) is identical to the previous one that isdescribed above, the orifice 119 being however eccentric depending onthe chosen angle. The shape of orifice 119 is defined depending on theclearances allowed with the rod 110.

The rod 110 is thus able, thanks to these new technical characteristics,to function in tension/compression with a given angle with respect tothe rod 116 or the rod 131 in the case in which the 119 orifice iseccentric and adjusted to the rod 116 or 131 (see FIG. 14).

The rod 110, forming an angle with respect to the rod 116 or 131 (thecase in which the 119 orifice is oblong and eccentric), can in this casefunction equally well in tension/compression as in lateral flexion. (seeFIG. 15).

The rod 110 can function in tension/compression and in flexion followinga preferred axis which can be, for instance, in the sagital plane of thespinal column, and this one on the one side and one the other side of agiven position of the rod 110 which, at rest, forms an angle with therod 116 or the rod 131, this also being the case where the component 119is oblong or eccentric, (FIG. 16).

Finally, the rod 110 can function in tension/compression and in flexionin all directions, forming an angle, as against the rod 116 or 131 incase the orifice 119 is eccentric or larger than the diameter of the rod110 (see FIG. 17).

Multiple varitions and modifications are possible in the embodiments ofthe invention described here. Although certain illustrative embodimentsof the invention have been shown and described here, a wide range ofmodifications, changes, and substitutions is contemplated in theforegoing disclosure. In some instances, some features of the presentinvention may be employed without a corresponding use of the otherfeatures. Accordingly, it is appropriate that the foregoing descriptionbe construed broadly and understood as being given by way ofillustration and example only, the spirit and scope of the inventionbeing limited only by the appended claims.

1. A flexible vertebral linking device, comprising: a cylindrical bodyportion having a first end and a second end; a first rod portionextending from the first end in a first direction from the cylindricalbody portion; a second rod portion having a first elongated body and anenlarged end portion, wherein the enlarged end portion is positionedwithin the cylindrical body portion and configured to pivot within thecylindrical body portion to adjust the alignment of the second rodportion with respect to a longitudinal axis of the cylindrical bodyportion, and wherein the first elongated body extends in a seconddirection opposite the first direction and passes through an opening inthe cylindrical body portion second end, the opening in the cylindricalbody portion second end having a width that is less than a width of theenlarged end portion but is greater than a width of the first elongatedbody; a first dampening member positioned between the enlarged endportion and the cylindrical body portion first end such that the firstdampening member does not encircle any portion of the second rodportion; and a second dampening member positioned between the enlargedend portion and the cylindrical body portion second end.
 2. The flexiblevertebral linking device of claim 1, wherein the cylindrical bodyportion second end comprises a cap.
 3. The flexible vertebral linkingdevice of claim 2, wherein the cap includes a threaded inner region. 4.The flexible vertebral linking device of claim 1, wherein the first rodportion includes a threaded end configured to engage a threaded portionof the cylindrical body portion.
 5. The flexible vertebral linkingdevice of claim 1, wherein the second dampening member includes a ringshape with an opening configured to receive the first elongated body ofthe second rod portion.
 6. The flexible vertebral linking device ofclaim 1, wherein the opening in the cylindrical body portion second endis circular.
 7. The flexible vertebral linking device of claim 1,wherein the opening in the cylindrical body portion second end iseccentrically located on the cylindrical body portion second end.
 8. Theflexible vertebral linking device of claim 1, wherein the opening in thecylindrical body portion second end includes an oblong shape having atleast one dimension that is greater than the width of the firstelongated body.
 9. A method of joining vertebral implants, comprising:providing a cylindrical body portion having a first end and a secondend; connecting a first rod portion extending from the first end in afirst direction; positioning a first dampening member within thecylindrical body portion; positioning a part of a second rod portionhaving a first elongated body and an enlarged end portion within thecylindrical body portion such that the first dampening member is locatedbetween the enlarged end portion and the cylindrical body first end andthe first dampening member does not encircle any portion of the secondrod portion; positioning a second dampening member between the enlargedend and the cylindrical body portion second end; and placing a caphaving an opening that includes a width that is less than a width of theenlarged end portion of the second rod portion and is greater than awidth of the first elongated body over the second rod elongated bodysuch that the elongated body passes through the opening and the enlargedend portion is secured within the cylindrical body portion, wherein theenlarged end portion is configured to pivot within the cylindrical bodyportion to adjust the alignment of the second rod portion with respectto a longitudinal axis of the cylindrical body portion.
 10. The methodof claim 9, wherein the cap includes a threaded inner region.
 11. Themethod claim 9, wherein the first rod portion includes a threaded endconfigured to engage a threaded portion of the cylindrical body portion.12. The method of claim 9, wherein the second dampening member includesa ring shape with an opening configured to receive the first elongatedbody of the second rod portion.
 13. The method of claim 9, wherein theopening in the cap is circular.
 14. The method of claim 9, wherein theopening in the cap is eccentrically located on the cylindrical bodyportion second end.
 15. The method of claim 9, wherein the opening inthe cap includes an oblong shape.